In a variety of fluid control systems, the flow of fluid through the system is controlled by flow control valves. In facilities that produce, distribute, or otherwise handle oil, natural gas, chemical products, or other fluids and/or gases, flow control valves are used to direct and regulate the flow of such process fluids in or through pipes, wells, pumps, vessels, refineries, or other equipment. Known flow control valves generally are shiftable between an open position that permits the flow of fluid, and a closed position that completely shuts-off the fluid flow. Other flow control valves are intended to regulate (e.g., throttle) the pressure and flow rate of the fluid flowing through the valve.
Due to high flow rates and high pressures in some systems, fluids passing through a valve or the control valve trim may experience cavitation or flashing, in which the flow dynamics cause the pressure to drop abrubtly, thus creating bubbles. Cavitation or flashing may generate an excessive amount of noise, and also may cause erosion and excessive wear on the surrounding components due to the abrasive nature of the fluid flow. The occurrence of cavitation or flashing, and the accompanying noise and/or vibration, can eventually reduce the performance of the valve and may even lead to failure of the valve.
In certain applications, a flow control valve may experience the phenomenon of out-gassing as the valve is opening. Out-gassing is the process by which gases dissolved in a fluid come out of solution due to a change in pressure. One way to illustrate this concept is to consider a container of carbonated beverage under pressure. At first glance, the beverage appears to be a homogeneous liquid. However, if the container is shaken and then opened, the dissolved CO2 comes out of solution and will spray out of the container.
In process control systems, due to the initial high pressure present on the inlet side of the closed valve, the fluid may experience a very high pressure drop as it passes the control element or valve plug upon opening the valve. This initial pressure drop across the valve may be high enough to allow any gas contained in the fluid to separate out.
As is known, out-gassing can damage a valve in a variety of ways. For example, the high velocity jets coming out of solution carry small liquid particles. These liquid particles can impinge on internal surfaces at very high velocities, thus causing erosion damage. The high velocity jets also can impinge on the surrounding valve wall or other components to create vibration, which also can damage the valve. Finally, when the gas separates out of the fluid, the volume and speed of the fluid increases, creating both noise and vibration.